Conventional liquid crystal devices are based on what may be called the “uniaxial nematic” liquid crystal phase, which has one optic axis. The typical molecules constituting the uniaxial nematic phase are roughly cylindrical molecules. In these devices, the optic axis, also known as the director, is oriented in a predetermined configuration using surface effects. Surface alignment films are prepared on two substrates to persuade the director to be parallel to one of the in-plane directions. The substrates are then put together to fabricate a nematic device in a number of different configurations. If the director orientation at the two surfaces is along orthogonal directions, it results in a twisted nematic (TN) device. If it is at 180° or 270°, it is called a high birefringence effect (HBE) or super twisted nematic (STN) device. Several other modes, e.g. parallel or OCB mode, are known and used.
The operation of such devices relies on producing different optical path-lengths for a beam of light passing through it. An applied electric field is applied by connecting a battery to the conducting electrodes predeposited at the two opposing substrates. The field exerts a torque on the director and tries to make it perpendicular to the substrates. During the change in director reorientation due to the applied field, the elongated LC molecules reorient about their short axis to change from ‘lying down’ at the substrate to ‘standing up’ position. Since the index of refraction parallel and perpendicular to the director are different. The “field-off” and “field-on” states appear different to a beam of propagating polarized light and its state of polarization is altered as it passes through such a device. An analyzer positioned at the exit side of the device at proper orientation is used to change the director reorientation in to optical contrast. This forms the principle of operation of LC display devices.
Speed of switching in the nematic devices depends on the elastic constants and the effective viscosity of the liquid crystal used. Generally, the switching speed lies in 10-100's of ms range. The speed of these devices has been one of the limiting factors and the major reason for needing more complex active matrix substrates. It is becoming obvious that faster devices are necessary to make significant advances and produce next generation of LC displays and electro-optical devices. The use of biaxial nematic liquid crystal lends itself to the fabrication of much faster devices.
The existence of a biaxial nematic liquid crystal phase possessing two orthogonal optic axes has previously been predicted. The recent discovery of novel mesophases formed by bent-core liquid crystals has encouraged scientific interest in the study of this phase and the possibility of the biaxial nematic phase being used for technological applications including display devices. In particular, computer simulation of such “boomerang” type molecules has suggested that the bent-core shape could exhibit a stable biaxial nematic phase. Recently, several studies have confirmed the existence of a stable biaxial nematic phase in several homologous series of liquid crystal compounds.
The molecules that are likely to form a biaxial nematic phase are made of ellipsoidal molecules that have, on average, three different dimensions in contrast to the cylindrical molecules that form the uniaxial nematic phase. They have two optic axes or directors, denoted as m and n (bold letter implies vector or directional nature). The advantage of such materials is that, in addition to reorientation of the principal director n, the secondary director m can also reorient. Reorientation of m requires rotation of its shorter axes about the long molecular axis, which can happen very fast. This is equivalent to a person turning on ones side while lying on a flat surface. Since the biaxial phase has three different indices of refraction in the three spatial directions, this turning (or, spinning) of molecules also offers a change in optical path length in the direction perpendicular to their length and thus a means to fabricate an electro-optical device.
It is now proposed herein that higher switching speeds in an electro-optic device can be realized using a liquid crystal exhibiting such a biaxial nematic phase.